1. Field of the Invention
The invention relates to a method of increasing the elasticity of moisture-cured elastomers comprising alkoxy-cross linking one-component compositions.
2. Background Art
Organic polymers with silane end groups in the form of one-component, moisture-curing compositions (RTV-1) are known and are much used for producing elastic sealants and adhesives. Polymers of this kind may be synthesized from different building blocks. Normally these are polyurethane, polyethers, polyesters, polyacrylates, polyvinyl esters, ethylene-olefin copolymers, styrene-butadiene copolymers or polyolefins. It is known that in order to provide stability during processing and storage of the compounded formulations, low molecular weight compounds possessing hydrolyzable groups, which have more enhanced reactivity with respect to water than the silane-terminated polymers, are added to these one-component compositions. The amount of added water scavengers is guided by the water content of the ingredients of the composition and by the desired storage stabilities and processing lives. Generally these are organo-functional silanes, the organic radical in many instances being critical to the reactivity. Examples of such silanes are vinyltrimethoxysilane, alkylaminopropyltrimethoxysilanes, but also, for example, silanes which bind water with concomitant formation of ammonia, for example hexamethyldisilazane.
Examples of alkoxysilane-terminated polymers with an organic backbone and processes for preparing crosslinkable compositions are described for example in EP-A-269 819, EP-A-370 464, EP-A-565 078, EP-A-931 800, WO 00/37533, U.S. Pat. No. 3,971,751 or DE-A-198 49 817. The same is true of organosiloxanes in a multiplicity of patents and publications (e.g., WO 99/06486).
Terminal groups in one component organic polymer systems are generally substituted propyltrimethoxysilanes, since the latter are generally readily available and exhibit very good reactivity in the compositions. These polymers, however, owing to their high reactivity, are also problematic in terms of processing, particularly in connection with incorporation of water-containing fillers or additives which further increase the reactivity. Additionally, the storage stabilities of the resulting compositions are frequently inadequate. For example, the addition of relatively large amounts of aminosilanes as adhesion promoters can greatly reduce the storage stability. The compositions generally have to be stabilized via further added components such as the phosphoric esters described in DE 19923300, in order to moderate the catalyst activity. Moreover, addition of standard water scavengers such as vinyltrimethoxysilane is only of limited aid in stabilizing the compositions.
More recent one-component systems use not only the propyl-spaced silanes, but also so-called alpha-silanes as crosslinkable groups for the end blocking of the polymers, as described for example in WO 02/066532 or WO 03/018658. In these systems it is also possible to install highly reactive difunctional or even monofunctional end groups as described in WO 03/059981, bringing substantial advantages with respect to elasticity.
A substantial disadvantage of many systems, is the divergent effect between the chain length and hence molecular weight of the polymer to be used, and the processing properties in terms of the viscosity. High molecular weights are of advantage not only on account of the resultant higher mechanical strength, but are also an important prerequisite for preparing low-modulus elastomers, as required particularly in sealants. If polymers of relatively low viscosity can be used in compositions of this kind, then establishing the processing properties becomes much easier and more flexible. This is a problem specifically in the case, inter alia, of silane-terminated polyurethanes. In such cases, it is virtually impossible to prepare low-modulus compositions without markedly impairing the mechanical properties as a result of necessary additions of plasticizer.